Non-contact ultrasonic tonometer

ABSTRACT

A non-contact ultrasonic tonometer for measuring intraocular pressure of an examinee&#39;s eye, in non-contact manner by use of an ultrasonic wave comprises: a probe including a vibrator for making the ultrasonic wave incident on the examinee&#39;s eye and a sensor for detecting the ultrasonic wave reflected from the examinee&#39;s eye; and an observation optical system for observing an anterior segment of the eye, wherein the probe is placed in an optical path of the observation optical system, and the observation optical system forms an image of the anterior segment through a surrounding region of the probe.

TECHNICAL FIELD

The non-contact ultrasonic tonometer for measuring intraocular pressureof an examinee's eye in non-contact manner by ultrasound.

BACKGROUND ART

There is proposed a contact intraocular pressure examination apparatusfor measuring intraocular pressure by pressing a distal end of a probepen against an eye, the probe pen holding a probe device including avibrator for introducing vibration into the eye and a vibrationdetecting sensor for detecting the vibration reflected by the eye (seePatent Literature 1: JP2004-267299A).

Also proposed is a non-contact intraocular pressure measurement systemfor measuring intraocular pressure by making an ultrasonic wave enter inan eye (actually, an eyeball model) and detecting the ultrasonic wavereflected from the eye by use of a sensor (see Non-patent Literature 1:“Development of a new non-contact intraocular pressure measurementsystem using a phase shift method”, Masayuki JINDE and other threepersons, Conference of Institute of Electrical Engineers, Sensors andMicromachines Division, Document p. 93-96, 2007). This system isarranged to measure a phase shift of a reflected wave with respect to atransmission wave as a frequency change, and determine a correlationbetween an amount of the frequency change and hardness of the eye model.

In the case of the apparatus configuration of Patent Literature 1,however, the probe pen is brought into contact with the eye to measureintraocular pressure and thus a large burden would be given to the eye.The apparatus configuration of Non-patent Literature 1 is merelyintended to measure the eyeball model, which is insufficient to measurehuman eyes. In the case of measuring human eyes, which exhibitinvoluntary eye movement and visual line movement, the ultrasonic wavecharacteristics (e.g., frequency and phase) detected by the sensor arelikely to vary due to misalignment of the apparatus with the eye,leading to variations in measurement results.

SUMMARY OF INVENTION Technical Problem

The present invention has a purpose to provide a non-contact ultrasonictonometer capable of easily making alignment of the tonometer withrespect to an examinee's eye.

Solution to Problem

To achieve the above purpose, the present invention provides anon-contact ultrasonic tonometer for measuring intraocular pressure ofan examinee's eye in non-contact manner by use of an ultrasonic wave,the tonometer comprising: a probe including a vibrator for making thenultrasonic wave incident on the examinee's eye and a sensor fordetecting the ultrasonic wave reflected from the examinee's eye; and anobservation optical system for observing an anterior segment of the eye,wherein the probe is placed in an optical path of the observationoptical system, and the observation optical system forms an image of theanterior segment through a surrounding region of the probe.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective external view of a non-contact ultrasonictonometer of a preferred embodiment of the present invention;

FIG. 2 is a perspective configuration view of a measurement system andan optical system of the tonometer;

FIG. 3 is a perspective configuration view (partly a block diagram) of acontrol system of the tonometer;

FIGS. 4A and 4B are views showing examples of an observation screendisplayed on a monitor;

FIG. 5 is a view showing a case where an objective lens is placed behinda probe;

FIG. 6 is a view showing a modified example of a fixation targetprojection optical system and a first alignment mark projection opticalsystem;

FIG. 7 is a view showing a case where the probe is displaced in adirection perpendicular to an optical axis of an observation opticalsystem;

FIG. 8 is a view showing a case where the probe is placed out of anoptical path of the observation optical system; and

FIG. 9 is a view showing a case where the tonometer is provided with aneye refractive power measurement optical system.

DESCRIPTION OF EMBODIMENTS

A preferred embodiment of the present invention will be described belowwith reference to accompanying drawings. FIG. 1 is a perspectiveexternal view of a non-contact ultrasonic tonometer 100 of thisembodiment.

The tonometer 100 is a so-called stationary apparatus including a base1, a head support unit 2 attached to the base 1, a movable unit 3movably placed on the base 1, and a measurement part 4 that is movablyprovided on the movable base 3 and contains a measurement system andoptical systems mentioned later. The measurement part 4 is moved in aright-and-left direction (an X-direction), an up-and-down direction (aY-direction), and a back-and-force direction (a working distancedirection; a Z-direction) relative to an examinee's eye E by a movementpart 6 provided in the movable unit 3. The movable unit 3 is moved inthe X- and Z-directions on the base 1 by inclining operation of ajoystick 5. The measurement part 4 is moved in the Y-direction by themovement part 6 by a rotating operation of a knob 5 a. The joystick 5 isprovided, at its top, with a measurement start switch 5 b. On themovable base 3, a monitor 72 is provided.

FIG. 2 is a perspective configuration view of the measurement system andoptical systems of the tonometer 100, showing a case where a probe isplaced in an optical path of an observation optical system. FIG. 3 is aperspective configuration view (partly a block diagram) of a controlsystem of the tonometer 100.

A probe (a transducer) 10 placed in front of the examinee's eye E has avibrator (an ultrasonic wave transmitting section) 11 for making anultrasonic wave (an incident wave, a transmission wave) incident on theeye E and a sensor (an ultrasonic wave receiving section) 13 fordetecting an ultrasonic wave (a reflected wave, a received wave)reflected by the eye E. The probe 10 is constituted of for example twopiezoelectric elements arranged one on another. One of the piezoelectricelements is used as the vibrator 11 and the other is used as the sensor13. In this embodiment, a pulse wave is used as the ultrasonic wave madeincident on the eye E but a continuous wave may be used instead.

On the side of the probe 10 closer to the eye E, an acoustic lens 16 isplaced to converge the ultrasonic wave from the vibrator 11. This lens16 comes into focus on the eye E when the probe 10 is placed in properalignment with the eye E.

The probe 10 is connected sequentially to an amplifier 81, a frequencycomponent analysis section 82, a frequency phase difference specifyingsection 83, and an arithmetic and control section 70. An electric signalcorresponding to an incident wave and a reflected wave is amplified toan appropriate signal level by the amplifier 81 and subjected tofrequency component analysis by the analysis section 82 to obtain aspectral distribution of the phase difference with respect to thefrequency. The specifying section 83 compares the spectral distributionof the incident wave and the spectral distribution of the reflected waveto specify a phase difference θx which is a difference in phase betweenthe incident wave and the reflected wave at respective frequencies fx.The phase difference θx at the frequency fx will vary according tointraocular pressure (strictly speaking, changes in hardness of a corneaof the examinee's eye E resulting from changes in the intraocularpressure). Accordingly, the arithmetic and control section 70 detectsthe phase difference θx based on an output signal of the specifyingsection 83 and obtains the intraocular pressure of the eye E based onthe detection result. This method is referred to U.S. Pat. No. 6,854,331(JP2002-272743A).

The probe 10 and the lens 16 are formed, at respective centers, with anaperture 18 (e.g., a circular hole having a diameter of about 1 mm)through which fixation target projecting light (hereinafter, referred toas “fixation target light”) from a light source 32 and alignment markprojecting light (hereinafter, referred to as “alignment mark light”)from a light source 42 are allowed to pass.

The vibrator 11 and the sensor 13 are electrically connected to acircuit system (the amplifier 81, the analysis section 82, thespecifying section 83, the arithmetic and control section 70, andothers) disposed out of an optical path of the observation opticalsystem 20 (hereinafter, referred to as an “observation optical path”)with a wiring cable 95. This cable 95 is covered with a cover 96 appliedwith a coating for absorbing reflected light (e.g., infrared light) froman anterior segment of the eye E. This makes it possible to prevent thereflected light by the anterior segment from diffusing on the surface ofthe cable 95 to be detected as noise light by a two-dimensional imagepickup device 26.

Provided as the optical systems of the tonometer 100 are the observationoptical system 20 for observing the anterior segment of the eye E, afixation target projection optical system 30 for causing the eye E tohold fixation, a first alignment mark projection optical system 40 forprojecting an alignment mark in the X- and Y-directions to the eye E, asecond alignment mark projection optical system 50 for projecting analignment mark in the Z-direction to the eye E, and an alignment markdetection optical system 55 for detecting the Z-direction alignment markprojected onto the eye E.

The observation optical system 20, having the optical path in which theprobe 10 is placed, forms an image of the anterior segment through aregion surrounding the probe 10. Specifically, the observation opticalsystem 20 includes, an objective lens 22, an imaging lens 24, a filter25, and the image pickup device 26 and provides an optical axis(hereinafter, an “observation optical axis”) L1 in which the probe 10 isplaced. Thus, when the observation optical axis L1 is aligned to apredetermined portion (for example, a corneal center or a pupil centerof the eye E), the probe 10 is placed in front of the eye E.Furthermore, in the configuration of FIG. 2, the probe 10 is arranged sothat the central axis (an extension of the central axis) of the probe 10is coaxial with the observation optical axis L1. Accordingly, when theobservation optical axis L1 is aligned with the predetermined portion ofthe eye E, the central axis of the probe 10 comes to coincide with thepredetermined portion of the eye E and therefore an ultrasonic wavereflected by the eye E can be efficiently detected.

Light sources 38 which emit infrared light to illuminate the anteriorsegment of the eye E are disposed diagonally to the front of the eye E.The filter 25 has a property of transmitting the light from each lightsource 38 and the light from the light source 42 and blocking light froma the light source 51.

The light from each light source 38 is projected onto the anteriorsegment of the eye E and then the light reflected by the anteriorsegment travels toward the lens 22. The light reaching the surroundingregion of the probe 10 passes through the lens 22, further passes a halfmirror 36 and a dichroic mirror 46, and forms an image on the imagepickup device 26 by the lens 24. Specifically, the anterior segmentimage by the light source 38 is formed on the image pickup device 26through the surrounding region of the probe 10. The dichroic mirror 46has a property of transmitting the light from each light source 38 andthe light from the light source 42 and reflecting the light from thelight source 32.

A signal output from the image pickup device 26 is input to thearithmetic and control section 70. The picked-up anterior segment imageis displayed on the monitor 72. In the configuration of FIG. 2, animaging optical system (a lens system constituted of a plurality oflenses) including the lenses 22 and 24 is used as a light deliverymember for delivering the anterior segment image to the image pickupdevice 26. Alternatively, a single lens may be used to deliver theanterior segment image to the image pickup device 26.

For a positional relationship between the probe 10 and the lens 22 inthe direction of the observation optical axis L1 (the Z-direction), thelens 22 is placed behind the probe 10 (closer to the image pickup device26) or in the same position as the probe 10 and hence wider space can beprovided between the eye E and a housing of the apparatus (see FIGS. 2and 5). In this case, the reflected light from the anterior segment maybe interrupted by the probe 10 and the lens 16. In order to allow clearobservation of the anterior segment image, the probe 10 and the lens 22are preferably placed close to each other on the observation opticalaxis L1 and more preferably placed in substantially the same position onthe observation optical axis L1.

FIG. 2 shows one example of the configuration in which the probe 10 andthe lens 22 are placed in the same position. That is, the lens 22 isformed, in almost the center thereof, with an aperture 22 a in which theprobe 10 is inserted. In this aperture 22 a, the probe 10 and the lens16 are set.

FIG. 5 shows one example of the configuration in which the lens 22 isplaced behind the probe 10. Specifically, the lens 22 is formed, inalmost the center thereof, with the aperture 22 a in which a supportmember 19 is inserted. The probe 10 and the lens 16 are set in a forwardposition in the support member 19 inserted in the aperture 22 a.

Alternatively, the lens 22 may be placed before the probe 10. In thiscase, the lens 22 is preferably formed with an aperture in a positioncorresponding to an ultrasonic wave propagation path in order to preventthe lens 22 from reflecting and attenuating the ultrasonic wave from theprobe 10. Thus, the incident wave from the vibrator 11 is allowed toenter the eye E through the aperture and the reflected wave from the eyeE is detected by the sensor 13 through the aperture.

The fixation target projection optical system 30 includes at least afixation target projection light source to project a fixation targetonto the eye E to cause the eye E to view the fixation target.Specifically, the projection optical system 30 includes the light source32, a fixation target 33, a diaphragm 34, a projection lens 35, and thedichroic mirror 46 to project the fixation target onto the eye E throughthe aperture 18. An optical axis L2 of the projection optical system 30is made coaxial with the observation optical axis L1 by the dichroicmirror 46 located in the observation optical path.

Light of the fixation target 33 illuminated by visible light from thelight source 32 is reduced in light diameter by the diaphragm 34, passesthrough the lens 35, is reflected by the dichroic mirror 46, passesthrough the half mirror 36, and then is projected onto the fundus of theeye E through the aperture 18. Thus, the eye E holds fixation.

The first alignment mark projection optical system 40 includes at leastan alignment mark projecting light source to project an alignment markin the X- and Y-directions onto the eye E from front. Specifically, theprojection optical system 40 includes the light source 42, a projectionlens 44, and the half mirror 36 to project the alignment marks (thealignment mark light) onto the eye E through the aperture 18. An opticalaxis L3 of the projection optical system 40 is made coaxial with theobservation optical axis L1 by the half mirror 36 located in theobservation optical path.

Infrared light from the light source 42 passes through the lens 44, isreflected by the half mirror 36, and then is projected onto the corneaof the eye E through the aperture 18. Light mirror-reflected by thecornea forms an image (an alignment mark image) i1 which is a virtualimage (a corneal reflection image) of the light source 42.

The light of the mark image i1 travels toward the lens 22. The lightreaching the surrounding region of the probe 10 passes through the lens22, the half mirror 36, and the dichroic mirror 46, and forms an imageon the image pickup device 26 by the lens 24. In other word, the markimage i1 by the light source 42 is formed on the image pickup device 26through the surrounding region of the probe 10. When the eye E moves inthe X- and Y-directions, an image forming position of the mark image i1also moves on the image pickup device 26. Based on a detection signal ofthe image pickup device 26, the arithmetic and control section 70 candetect an alignment state of the apparatus (the probe 10) in the X- andY-directions with respect to the eye E.

In the case of projecting the alignment mark light through the aperture18 formed in the probe 10 and the lens 16 and receiving the reflectedlight from the eye E through the surrounding region of the probe 10,part of the reflected light may be interrupted by the lens 16. To avoidsuch defect, as shown in FIG. 2, the projection optical system 40 ispreferably configured as an optical system whereby the alignment marklight is converged once before reaching the eye E and then is projectedas dispersion light onto the cornea.

The second alignment mark projection optical system 50 includes at leastan alignment mark projecting light source to project an alignment markin the Z-direction onto the eye E from an oblique direction.Specifically, the projection optical system 50 includes the light source51 and a projection lens 52 to project the alignment mark (the alignmentmark light) onto the eye E. An optical axis L4 of the projection opticalsystem 50 intersects with the observation optical axis L1 at apredetermined angle.

Infrared light from the light source 51 passes through the lens 52, issubstantially collimated, and then is projected onto the cornea of theeye E. The light mirror-reflected by the cornea forms an image (analignment mark image) i2 which is a virtual image (a corneal reflectionimage) of the light source 51.

The alignment mark detection optical system 55 includes aphoto-receiving lens 56, a filter 57, and a position sensitive device 58(e.g., a line CCD) to detect the alignment mark image formed by theprojection optical system 50. The filter 57 has a property oftransmitting the light from the light source 51 and blocking the lightfrom the light source 38 and the light from the light source 42. Anoptical axis L5 of the detection optical system 55 is symmetrical to theoptical axis L4 of the projection optical system 50 with respect to theobservation optical axis L1. The optical axis L5 intersects with theoptical axis L4 at a point on the optical axis L1.

The mark image i2 by the light source 51 is formed on the positionsensitive device 58 by the lens 56. When the eye E moves in theZ-direction, an image forming position of the mark image i2 moves on theposition sensitive device 58. Based on a detection signal of theposition sensitive device 58, the arithmetic and control section 70 candetect an alignment state of the apparatus (the probe 10) in theZ-direction with respect to the eye.

The arithmetic and control section 70 is coupled to the knob 5 a, theswitch 5 b, the movement part 6, the monitor 72, the specifying section83, the light sources 32, 38, 42, and 51, the image pickup device 26,the position sensitive device 58, an operation section (an inputsection) 74 provided with various switches, a memory 75 serving as astorage section, and others. The arithmetic and control section 70performs control of the entire apparatus, calculation of measuredvalues, and so on.

The memory 75 stores a table showing a correlation between the phasedifference θx at the frequency fx and an intraocular pressure value. Thearithmetic and control section 70 retrieves an intraocular pressurevalue corresponding to the detected phase difference θx from the memory75 based on the output signal of the specifying section 83 and displaysthe retrieved intraocular pressure value on the monitor 72.

The correlation between the phase difference θx and the intraocularpressure value can be set by experimentally determining in advance acorrelation between phase differences θx obtained by the presentapparatus and intraocular pressure values measured by a Goldmanntonometer. The memory 75 stores a program for measuring intraocularpressure by use of the probe 10, a program for controlling the entireapparatus, and so on.

On the operation part 74, there are arranged a selection switch 74 a forselecting either an automatic alignment mode of automatically aligningthe measurement part 4 with respect to the eye E or a manual alignmentmode of manually aligning the measurement part 4 with respect to the eyeE, a selection switch 74 b for selecting either an automatic shot modeof automatically generating a trigger signal to start measurement uponcompletion of alignment or a manual shot mode of generating a triggersignal to start measurement based on an operation signal of the switch 5b, and others. When the automatic shot mode is selected, the arithmeticand control section 70 determines whether the alignment state is properor not based on each detection signal of the image pickup device 26 andthe position sensitive device 58. Based on the determination result, thearithmetic and control section 70 generates a measurement-start triggersignal and, based on the generation of the trigger signal, causes theprobe 10 to emit an ultrasonic wave to the eye E.

Operations of the apparatus having the above configuration are explainedbelow. Firstly, the face (head) of an examinee is fixed on the headsupport unit 2. An examiner makes alignment of the apparatus with theexaminee's eye E by manipulating the joystick 5 while viewing themonitor 72. At that time, the arithmetic and control section 70 displaysthe anterior segment image picked up by the image pickup device 26 and areticle LT and an indicator G for alignment on the monitor 72 as shownin FIGS. 4A and 4B.

When the mark image i1 starts to appear on the monitor 72 (when theimage pickup device 26 starts to detect the mark image i1), theautomatic alignment in the X- and Y-directions is enabled. Furthermore,when the position sensitive device 58 starts to detect the mark imagei2, the automatic alignment in the Z-direction is enabled. Thearithmetic and control section 70 controls display of the indicator Gbased on information about the alignment state in the Z-directionobtained from the detection signal of the position sensitive device 58.

The case of selecting the automatic alignment mode and the automaticshot mode is explained below. The arithmetic and control section 70obtains misalignment amounts of the apparatus in the X-, Y-, andZ-directions relative to the eye E located in a proper position, andcontrols driving of the movement part 6 to bring each misalignmentamount into a predetermined permissible range. When each misalignmentamount falls within the permissible range, the arithmetic and controlsection 70 stops the driving of the movement part 6 and automaticallygenerates the measurement-start trigger signal to start intraocularpressure measurement.

The case of selecting the manual alignment mode and the manual shot modeis explained below. In this case, the examiner manipulates the joystick5 (the knob 6 a) so that the mark image i1 displayed on the monitor 72enters in the reticle LT and the indicator G appears in the formrepresenting alignment completion (see FIG. 4B). When the alignment iscompleted in each direction and the switch 5 b is pressed by theexaminer, the arithmetic and control section 70 generates themeasurement-start trigger signal to start the intraocular pressuremeasurement.

Upon generation of the measurement-start trigger signal, the arithmeticand control section 70 causes the vibrator 11 to emit the ultrasonicwave to the eye E and detects the ultrasonic wave reflected from the eyeE by the sensor 13. The arithmetic and control section 70 calculates anintraocular pressure value of the eye E based on the output signal ofthe specifying section 83 and displays a result thereof on the monitor82.

With the above configuration, the alignment between the eye E and theprobe 10 can be easily performed.

In the above explanation, the fixation target light is projected ontothe eye. E through the aperture 18 formed in the center of the probe 10.It is not limited thereto but may be arranged to project the fixationtarget light through the surrounding region of the probe 10 in the lens22. A conceivable configuration in this case is, for instance, to use adiaphragm having an annular aperture centered on the optical axis L2instead of the diaphragm 34 having a spot aperture on the optical axisL2.

In the above embodiment, the alignment mark light is projected onto theeye E through the aperture 18 formed in the center of the probe 10. Itis not limited thereto but may be arranged to project the alignment marklight through the surrounding region of the probe 10 in the lens 22, andallow the image pickup device 26 to detect the reflected light passingthrough the surrounding region of the probe 10 in the lens 22. Aconceivable configuration in this case is, for instance, to use anannular light source instead of the spot light source 42.

FIG. 6 is a view showing a modified example of the fixation targetprojection optical system and the first alignment mark projectionoptical system. In this case, the light source 32 (e.g., an LED) isplaced in the center of the probe 10.

A first alignment mark projection optical system 140 for projectingalignment mark light at a predetermined angle to the observation opticalaxis L1 is placed outside the lens 22. Reflected light thereof isallowed to pass through the surrounding region of the probe 10 in thelens 22. In this case, the angle of the optical axis of the projectionoptical system 140 to the observation optical axis L1 is determined toprevent part of the reflected light from becoming interrupted by thelens 16.

In the case where the light source 32 is placed in the center of theprobe 10 as shown in FIG. 6, a light source that emits visible light andinfrared light may be used as the light source 32 to serve both as thefixation target projecting light source and the alignment markprojecting light source.

In the above explanation, the probe 10 is placed on the observationoptical axis L1 but not limited thereto. The probe 10 may be displacedfrom the observation optical axis L1 in a direction (the X- andY-directions) perpendicular to the optical axis L1 as shown in FIG. 7.In this case, a detection position of the mark image i1 on the imagepickup device 26 when the central axis (the extension of the centralaxis) of the probe 10 comes into alignment with the predeterminedportion (e.g., the corneal center or the pupil center) of the eye E isset as an alignment reference position, and a display position of thereticle LT, an alignment completion position, and others are set.

FIG. 8 is a schematic configuration view of the measurement system andthe optical system of the tonometer 100, showing the case where theprobe is placed out of the optical path of the observation opticalsystem.

An ultrasonic wave reflecting member (an acoustic mirror) 90 reflects anincident wave from the vibrator 11 toward the eye E while reflecting areflected wave from the eye E toward the sensor 13. The observationoptical system 20 is arranged so that the probe 10 is placed out of theoptical path thereof and the observation optical axis L1 is positionedon an ultrasonic wave propagation path between the reflecting member 90and the eye E. The lens 22 is formed with an aperture 22 b through whichan ultrasonic wave from the probe 10 is allowed to pass. The incidentwave from the vibrator 11 is reflected by the reflecting member 90 toenter the eye E after passing through the aperture 22 b. The reflectedwave from the eye E passes through the aperture 22 b, is reflected bythe reflecting member 90, and then is detected by the sensor 13.

In the case where the lens 22 is placed between the reflecting member 90and the eye E, the lens 22 formed with the aperture 22 b in a portioncorresponding to the ultrasonic wave propagation path can avoidattenuation of the ultrasonic wave which is likely to be caused inpassing through the lens 22. In this case, the reflecting member 90applied with a coating having a property of blocking the reflectionlight from the anterior segment by the light source 38 may be used toprevent the anterior segment reflection light from entering the imagepickup device 26 through the aperture 22 b, thereby preventing resultantnoise light.

As the reflecting member 90, a member having a property of reflectingthe ultrasonic wave and transmitting light (for example, a transparentand colorless, hard plastic plate) may be used. This can prevent thefixation target light and the alignment mark light from becominginterrupted even when the reflecting member 90 is placed in each opticalpath of the projection optical system 30 and the projection opticalsystem 40. In the case of using the reflecting member 90 having a lighttransmission property, taking into consideration that the optical lengthis changed by passage of light through the reflecting member 90, amember having the area almost equal to an optical path splitting membersuch as the half mirror 36 and the dichroic mirror 46 may be used.

The present invention is not limited to the above configuration and maybe arranged such that the reflecting member 90 is partly provided withan aperture through which the fixation target light and the alignmentmark light are allowed to pass to be projected onto the eye E. The aboveconfiguration shows the case in which the reflecting member 90 is placedin a common optical path of the projection optical systems 30 and 40.The above configuration can be applied to the case where the reflectingmember 90 is placed in the optical path of at least one of theprojection optical systems 30 and 40.

The configuration in which the probe 10 is placed out of the observationoptical path is not limited to one shown in FIG. 8 and may be arrangedso that the reflecting member 90 is placed between the lens 22 and theeye E. In this case, the incident wave from the vibrator 11 is reflectedby the reflecting member 90 to enter the eye E, while the reflected wavefrom the eye E is reflected by the reflecting member 90 and detected bythe sensor 13.

The tonometer may be additionally provided with a measurement opticalsystem for measuring eye characteristics different from the intraocularpressure. FIG. 9 is a view showing the case where an eye refractivepower measurement optical system is added to the tonometer.

An eye refractive power measurement optical system 310 is arranged sothat the probe 10 is placed out of an optical path of the measurementoptical system 310 (a measurement optical path), and an optical axis L6of the measurement optical system 310 (hereinafter, referred to as a“measurement optical axis”) is located on the ultrasonic wavepropagation path between the reflecting member 90 and the eye E. Thereflecting member 90 is placed in front of the eye E. The incident wavefrom the vibrator 11 is reflected by the reflecting member 90 to enterthe eye E and the reflected wave from the eye E is reflected by thereflecting member 90 and detected by the sensor 13. Thus, theintraocular pressure of the eye E is measured.

The measurement optical system 310 is placed on the transmission side ofa dichroic mirror 301 located at the rear of the reflecting member 90.The measurement optical system 310 is an optical system for projectingmeasurement light to the fundus of the eye E and receiving reflectedlight from the fundus by a photo-receiving device. Based on an outputsignal of the photo-receiving device, the eye refractive power ismeasured. The measurement optical system 310 and a measurement principleof eye refractive power are well known and thus their details areomitted herein.

On the reflection side of the dichroic mirror 301, an objective lens311, a dichroic mirror 312, and a total reflection mirror 313 areplaced. On the reflection side of the mirror 313, a fixation targetprojection optical system not shown is arranged to cause the eye E toview the fixation target.

On the reflection side of the dichroic mirror 312, arranged is anobservation optical system 322 including an imaging lens 320 and atwo-dimensional image pickup device 321 placed in a substantiallyconjugate relationship with the vicinity of the anterior segment of theeye E. The image pickup device 321 picks up the anterior segment imageformed by a light source 325 and a mark image formed by the alignmentmark projection optical system not shown.

The measurement optical axis L6 and an optical axis L7 of theobservation optical system 322 are made coaxial by the dichroic mirror301. The dichroic mirror 301 has a property of transmitting light from alight source of the measurement optical system 310 and reflecting thelight from the light source 325, light from a light source of thealignment mark projection optical system, and light from a light sourceof the fixation target projection optical system. The dichroic mirror312 also has a property of transmitting the light from the light sourceof the fixation target projection optical system and reflecting thelight from the light source 325 and the light from the light source ofthe alignment mark projection optical system. Used as the reflectingmember 90 is a member having a property of reflecting an ultrasonic waveand transmitting light (e.g., a transparent and colorless, hard plasticplate). This member transmits the measurement light by the measurementoptical system 310, the anterior segment reflected light by the lightsource 325, the fixation target light by the fixation target projectionoptical system, the alignment mark light by the alignment markprojection optical system, and others.

In the configuration of FIG. 9, the reflecting member 90 is placed in acommon optical path of the measurement optical system 310, theobservation optical system 322, and the fixation target projectionoptical system but it is not limited thereto. The configuration has onlyto reflect the ultrasonic wave from the probe 10 by the reflectingmember 90 to enter the eye E from front. For instance, the reflectingmember 90 may be placed between the dichroic mirror 301 and the lens311.

Although the above explanation exemplifies the eye refractive powermeasurement optical system, the present invention is not limited theretoand may be applied to a measurement optical system for measuring eyecharacteristics different from intraocular pressure by receivingreflected light resulting from measurement light projected onto the eyeE. For example, a non-contact type eye axial length measurement opticalsystem (e.g., see U.S. Pat. No. 7,434,932 (JP2007-37984A), a cornealthickness measurement optical system (e.g., see JP63-197433 (1988)A),and others may be adopted.

Furthermore, a mode of performing intraocular pressure measurement basedon the measurement-start trigger signal may be selected as needed with aswitch or the like between a normal measurement mode of performing oneintraocular pressure measurement in response to one trigger signal and acontinuous measurement mode of repeating intraocular pressuremeasurements several times in response to one trigger signal.

The case of selecting the continuous measurement mode is explainedbelow. Upon generation of the measurement-start trigger signal, thearithmetic and control section 70 causes the probe 10 to continuouslyemit an ultrasonic wave pulse toward the eye E to obtain information onvariations in intraocular pressure caused by pulsation of the eye E, andperforms the arithmetic processing corresponding to each ultrasonic wavepulse continuously emitted.

Specifically, the ultrasonic wave pulse is continuously made incident onthe eye E at predetermined time intervals (e.g., 0.1 seconds intervals)within a range (e.g., within 1.5 seconds) of a pulsation cycle of theeye E, and an intraocular pressure value corresponding to eachultrasonic wave pulse is calculated. In this way, many intraocularpressure values can be obtained within the range of the pulsation cycleand thus variations in intraocular pressure values in the pulsationcycle can be captured. In this case, based on each measured valueobtained within the range of the pulsation cycle, it is possible tocalculate a representative value (e.g., an average value of the measuredvalues, a central value of the measured values) and calculate measuredvalues at a peak, a bottom, and a middle of the pulsation.

In the above explanation, the ultrasonic wave pulse is emitted at thepredetermined time intervals (e.g., 0.1 seconds intervals) but notlimited thereto. The ultrasonic wave pulse may be emitted at apredetermined number of times previously set within the range of thepulsation cycle. The time intervals and number of emissions forcontinuously emitting the ultrasonic wave pulse may be made arbitrarilysettable and a switch thereof may be provided in the operation part 74.

In the above explanation, the alignment state of the apparatus withrespect to the eye E in the Z-direction is optically detected (a workingdistance is detected) but it may be detected by the probe 10 used forintraocular pressure measurement. In this case, the control of the probe10 has to be switched between control for measuring the intraocularpressure and control for detecting the working distance. In the case ofdetecting the working distance with respect to the eye E by use of theprobe 10, the arithmetic and control section 70 measures a measurementtime T from emission of the incident wave from the vibrator 11 towardthe eye E until the reflected wave from the eye E is detected by thesensor 13 and thereby detects the working distance of the probe 10 fromthe eye E. In other words, as the measurement time T from the emissionof the ultrasonic wave from the vibrator 11 until the ultrasonic wave isdetected by the sensor 13 is longer, the working distance is larger. Asthe measurement time T is shorter, the working distance is smaller. Thearithmetic and control section 70 previously determines a referencemeasurement time Tk for which the working distance is proper withrespect to the eye E and considers the alignment in the Z-direction tobe completed when the measurement time T reaches the referencemeasurement time Tk.

In the case of the above configuration, for example, the arithmetic andcontrol section 70 controls the probe 10 as a working distance sensorwith respect to the eye E before completion of alignment and controlsthe probe 10 as an intraocular pressure measurement sensor with respectto the eye E after completion of alignment. This can facilitate theconfiguration for detecting the alignment state of the apparatusrelative to the eye E in the Z-direction.

The above explanation is made to determine the intraocular pressurebased on a difference in acoustic impedance resulting from the phasedifference between an input phase and an output phase. The presentinvention is not limited thereto and may be applied to a configurationthat can determine intraocular pressure by performing a comparison andarithmetic processing of the incident wave from the vibrator 11 and thereflected wave detected by the sensor 13. For instance, it may bearranged to determine intraocular pressure by performing a comparisonand arithmetic processing of the frequency of the incident wave from thevibrator 11 and the frequency of the reflected wave detected by thesensor 13. Specifically, a phase shift circuit may be provided to shiftthe phase difference to zero by changing the frequency of an ultrasonicwave generated by the vibrator 11 when the phase difference occursbetween the input waveform to the vibrator 11 and the output waveformfrom the sensor 13. The intraocular pressure is determined by detectinga frequency change amount when the phase difference is shifted to zero.

1. A non-contact ultrasonic tonometer for measuring intraocular pressureof an examinee's eye in a non-contact manner by use of an ultrasonicwave, the tonometer comprising: a fixation target projection opticalsystem provided with a light source for fixation target projection toproject a fixation target onto the eye to cause the eye to view thefixation target; a probe including a vibrator for making the ultrasonicwave incident on the examinee's eye and a sensor for detecting theultrasonic wave reflected from the examinee's eye, the probe beingconfigured to measure the intraocular pressure based on an output signalfrom the sensor; and an observation optical system for observing ananterior segment of the eye, wherein the probe is placed in an opticalpath of the observation optical system and has an aperture through whichfixation target light from the light source is allowed to pass, and theobservation optical system forms an image of the anterior segmentthrough a surrounding region of the probe.
 2. The tonometer according toclaim 1 further comprising an alignment mark projection optical systemprovided with a light source for alignment mark projection to project analignment mark onto the eye, and an alignment mark detection opticalsystem provided with an image pickup device to detect a cornealreflection image formed by the light source, wherein the observationoptical system forms the corneal reflection image on the image pickupdevice through the surrounding region of the probe.
 3. The tonometeraccording to claim 2, wherein the probe has an aperture through whichalignment mark light from the light source is allowed to pass.
 4. Anon-contact ultrasonic tonometer for measuring intraocular pressure ofan examinee's eye in a non-contact manner by use of an ultrasonic wave,the tonometer comprising: a fixation target projection optical systemprovided with a light source for fixation target projection to project afixation target onto the examinee's eye to cause the eye to view thefixation target; a probe including a vibrator for making the ultrasonicwave incident onto the examinee's eye and a sensor for detecting theultrasonic wave reflected from the examinee's eye, the probe beingconfigured to measure the intraocular pressure based on an output signalfrom the sensor; an observation optical system for observing an anteriorsegment of the eye; and a refractive power measurement optical systemconfigured to project measurement light onto a fundus of the examinee'seye and cause a photo-receiving device to receive reflection light fromthe fundus, the refractive power measurement optical system beingarranged to measure refractive power of the examinee's eye based on anoutput from the photo-receiving device, wherein the probe is placed outof an optical path of the fixation target projection optical system, theobservation optical system, the refractive power measurement opticalsystem.
 5. The tonometer according to claim 1, further comprising anacoustic lens placed on a side of the vibrator closer to the examinee'seye and for converging the ultrasonic wave from the vibrator, theacoustic lens including an aperture through which fixation target lightfrom the light source is allowed to pass, and the observation opticalsystem forms the anterior segment image through the surrounding regionof the acoustic lens.
 6. The tonometer according to claim 1, furthercomprising an alignment detection unit for detecting an alignment stateof the probe in a working distance direction with respect to theexaminee's eye.
 7. The tonometer according to claim 6, wherein thealignment detection unit includes a projection optical system forprojecting an alignment mark onto a cornea of the examinee's eye from anoblique direction and a detection optical system for detecting thealignment mark projected on the cornea from an oblique directionsymmetrical to the oblique projecting direction.
 8. The tonometeraccording to claim 1, wherein the probe is connected to a circuit systemwith a cable having a surface property of absorbing reflection lightfrom the anterior segment.
 9. The tonometer according to claim 4,further comprising an acoustic lens placed on a side of the vibratorcloser to the examinee's eye and for converging the ultrasonic wave fromthe vibrator.
 10. The tonometer according to claim 4, further comprisingan alignment detection unit for detecting an alignment state of theprobe in a working distance direction with respect to the examinee'seye.
 11. The tonometer according to claim 10, wherein the alignmentdetection unit includes a projection optical system for projecting analignment mark onto a cornea of the examinee's eye from an obliquedirection and a detection optical system for detecting the alignmentmark projected on the cornea from an oblique direction symmetrical tothe oblique projecting direction.